Hybrid organic inorganic perovskites (HOIPs) have started to emerge as leading materials for optoelectronic technologies including solar cells, photodetectors, lasers, and light‐emitting diodes. Within this family, two-dimensional (2D) layered HOIPs have attracted growing attention in recent years owing to their intrinsically superior environmental stability compared to most 3D HOIPs, as well as their remarkable structural and compositional tunability. The choice of organic ammonium cations, drawn from a vast library of candidate molecules, governs the assembly of these hybrid materials. Typically, this organic component does not directly influence the hybrid’s optical or electronic behavior; however, the incorporation of so-called electroactive organic cations has begun to receive considerable research interest.[1] For example, through the incorporation of such tailored organic cations, 2D HOIPs with an extended absorption spectrum, enhanced (out-of-plane) charge carrier transport, and reduced exciton binding energy have been obtained.

In 2023, we [2] showed that carbazole-based organic ammonium cations with different alkyl spacer lengths can be used to tune the optical and electronic properties of 2D lead iodide HOIPs. With decreasing spacer length, there was evidence for enhanced electronic coupling between the organic and inorganic layers. For all spacer lengths (3, 4, and 5 carbon spacers), light-induced charge transfer from the organic to the inorganic layer was detected. Specifically for the carbazole with the shortest spacer (Cz-3), an organic-inorganic (interlayer) charge transfer state was observed. The out-of-plane charge carrier transport was enhanced for all the carbazole-containing 2D HOIPs compared to that of the reference 2D HOIP containing an electronically inactive phenylethylammonium (PEA) cation, with the 2D HOIP based on Cz-3 possessing the highest charge carrier mobility.

In recent research, we build further on this work to gain a deeper understanding of the influence of molecular design on the optical and electronic properties of low-dimensional HOIPs. We compare the properties of two low-dimensional hybrids containing the same carbazole-inspired organic cation but with a different lead iodide inorganic framework. Depending on the connectivity of the octahedra, differences in the photoinduced charge transfer dynamics between the organic and inorganic layers are obtained, and other transfer pathways become available because of changes in relative energy alignment between organic and inorganic states. In other recent work, we studied 2D HOIPs containing electroactive organic cations for which we were able to determine the crystal structures to deduce detailed structure-property relationships. In a combined experimental-computational study, we show that the organic-inorganic interlayer electronic coupling is highly sensitive to the orientation of the organic core with respect to the inorganic framework.